1. Field of the Invention
The present invention relates to a photodetector for quantitatively measuring weak light and, more particularly, to an electron tube having a photocathode and a sensing device for detecting photoelectrons emitted from the photocathode.
2. Related Background Art
Conventionally, an electron tube which causes an electron lens to accelerate and focus photoelectrons emitted from a photocathode and makes the photoelectrons incident on a sensing device, e.g., a semiconductor device to obtain a high gain, is known. A conventional electron tube is disclosed in, e.g., Japanese Patent Laid-Open No. 6-318447, U.S. Pat. Nos. 5,120,949 or 5,374,826, or G. A. Johansen, "Operational characteristics of an electron-bombarded silicon-diode photomultiplier tube", Nuclear Instruments and Methods In Physics Research A326 (1993), pp. 295-298.
Particularly, Japanese Patent Laid-Open No. 6-318447 or U.S. Pat. No. 5,374,826 discloses an electron lens constituted by a plate electrode having an opening portion for passing photoelectrons. On the other hand, U.S. Pat. No. 5,120,949 or reference by G. A. Johansen discloses an electron lens constituted by a cylindrical electrode having an opening portion for passing photoelectrons.